1. Field of the Invention
The present invention relates to an apparatus and a method for detecting a decrease in a tire air pressure, and a program for detecting a decrease in a tire air pressure.
2. Description of the Related Art
Conventionally, there has been an apparatus for detecting a decrease in a tire air pressure for detecting, based on the rotation (wheel speed) information regarding four tires attached to a vehicle, a decreased pressure of a tire. This apparatus uses a principle according to which a tire having a decreased pressure shows a reduced outer diameter (tire dynamic loaded radius) compared to that of tires having a normal air pressure and thus shows an increased rotation velocity and an increased angular velocity compared to those of other normal tires. In the case of a method of detecting a reduced internal pressure based on a relative difference in the angular velocity of tires (see Japanese Unexamined Patent Publication No. 305011/1988 for example) for example, a judgment value DEL obtained by DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%) is used. In the formula, F1 to F4 represent angular velocities of a left front wheel tire, a right front wheel tire, a left rear wheel tire, and a right rear wheel tire, respectively.
However, since this method judges a decreased pressure based on the difference between the sum of angular velocities of two diagonally positioned tires of four wheels and the sum of angular velocities of the other two diagonally positioned tires, this method cannot judge a case where two front tires simultaneously have a decreased pressure or a case where two rear tires simultaneously have a decreased pressure. This causes a disadvantage where a driver continues to drive the vehicle while not knowing that the tires have a decreased pressure and thus the fuel consumption is deteriorated due to an increased rolling resistance of the tires and the tires may burst.
Thus, various methods have been suggested in order to judge a case where two front tires or two rear tires simultaneously have a decreased pressure as described above. In the case of Japanese Unexamined Patent Publication No. 2003-165318 for example, the rotation information of two front tires of a vehicle is compared with the rotation information of two rear tires to judge, based on the relation between the resultant comparison values and a predetermined threshold value, whether the two front tires or two rear tires simultaneously have a decreased pressure or not.
However, the conventional methods including the method according to Japanese Unexamined Patent Publication No. 2003-165318 are limited in that two front tires or two rear tires simultaneously having a decreased pressure can be detected only under limited vehicle running conditions. Specifically, the method according to Japanese Unexamined Patent Publication No. 2003-165318 judges that two front tires or two rear tires simultaneously have a decreased pressure only when the vehicle runs on a straight line and the tires are not driven or braked. Thus, this method cannot judge simultaneous decreased air pressure of two front tires or two rear tires when the vehicle is accelerated or decelerated.